Loudspeakers are essentially transducers which convert electrical energy into physical, acoustical energy. The design of typical basic loudspeakers has not changed for decades. Generally, a loudspeaker driver consists of a frame or housing, a cone or other diaphragm attached to a voice coil, a surround and spider suspension and a permanent magnet. Sound is created by moving the diaphragm to create sound waves in the air around the diaphragm. This is accomplished through electromagnetic attraction and repulsion of the voice coil. The outer periphery of the diaphragm is connected to the housing or frame by a flexible surround which allows the diaphragm to move freely and helps somewhat to keep the diaphragm and voice coil in proper alignment. The voice coil is typically a coil of wire which forms an inductor. As electrical current passes through the coil it produces a magnetic field. The voice coil is placed in close proximity to a permanent magnet which provides a permanent magnetic field which react with the variable magnetic field of the coil thereby causing the coil to be repelled or attracted according to the field of the coil and the polarity and magnitude of the coil current. The spider and surround keep the coil in precise alignment with the permanent magnet so that minute changes in current in the coil can accurately produce diaphragm movement and sound.
The physical characteristics of drivers can make them more suitable for reproducing sounds in certain frequency ranges. High frequency sound requires a driver that can react quickly, but which does not need a diaphragm that must displace a substantial distance. Low frequency sound requires a driver that can displace longer distances, but which does not need to react as quickly. Consequently, larger drivers, called woofers, are typically used to reproduce low frequency sound while very small, rigid drivers, called tweeters, are used for high frequency sound. A high-quality loudspeaker will generally have multiple drivers for reproducing sound in a variety of frequency ranges. Many loudspeakers will have at least a woofer, midrange and a tweeter to reproduce the entire audible sound spectrum, however, as the following disclosure will reveal, this can be achieved in other ways.
One problem inherent in typical driver design is the “backwave” created when the diaphragm rebounds from an extended position. This creates a sound wave which emanates from the back of the diaphragm which, if not controlled, may interfere with and even cancel the primary sound wave created by the diaphragm.
One method of dealing with backwave interference is to mount the driver in a sealed enclosure that will absorb the majority of the backwave preventing it from reaching the listener. This is commonly known as an “acoustic suspension” speaker. Another popular method of dealing with backwave emissions is to allow part of the wave to reach the listening area through a vent or port. This is known as a “bass reflex” design. Yet another method involves the use of a passive radiator or “drone driver” which vibrates with the backwave thereby absorbing energy and helping eliminate the backwave. All of these methods help somewhat to eliminate backwave interference, however they do so at the cost of lost energy and performance.
Backwave interference can also be dealt with using a bipolar speaker configuration. The typical bipolar configuration utilizes two identical drivers which are mounted in the front and back of a speaker enclosure. These two drivers are driven in-phase so that identical waves are emitted from the front and back of the enclosure. This eliminates the backwave cancellation problem because the waves are in-phase, but the drivers can suffer from a decreased response and lost energy due to the need to overcome increased pressure in the enclosure.
Another problem inherent with woofers which must move fair distances in order to reproduce low frequencies and large outputs is that of inertia. Once a driver diaphragm is displaced it must return to a neutral position before subsequent displacement. Inertia makes stopping a diaphragm at a neutral position difficult after a substantial displacement. Ideally, a woofer would need to increase its mechanical impedance as the distance from its neutral, or static, position increases. However, even if a driver is designed to near mechanical perfection, with the restorative force being equal to that of the initial current, stopping the driver at the “neutral” position remains a challenge.
An additional problem with current speaker technology is caused by misalignment of the voice coil with the permanent magnet due to distortion of the diaphragm or cone. Driver surrounds and spiders must be flexible to provide the necessary response to electrical input, but this makes the driver diaphragm extremely susceptible to unequal air pressure across its surface area. As a diaphragm encounters unequal air pressure due to enclosure discontinuities or air flow patterns, the diaphragm distorts causing the attached voice coil to rotate off its central axis. This causes the precisely balanced magnetic fields of the permanent magnet and the voice coil to misalign thereby causing an inductive variance and increased current draw from the amplifier. This results in decreased power handling, poorer response and inaccurate reproduction of sound.
What is needed is an apparatus and method for controlling driver backwaves and the air pressure and flow at the rear of the driver.
Backwave and air pressure problems are complicated by the fact that while a build-up of pressure is deleterious to linear operation, a certain amount of back pressure can help control driver inertial problems. The helpful portion of the back pressure relates directly to the mechanical movement of the driver and is purely an attempt to control over-excursion. Hence, a decrease in cross sectional area allows for a measure of pressure build-up. The minimum cross sectional area being that of the radiating area of the driver in question helps to ensure that modulation through pressure build-up is kept to a minimum.